In the continuous casting of steel, molten metal may be delivered to a mold by means of an upper tundish nozzle attached to the bottom of a tundish, a slide gate plate assembly below the upper tundish nozzle and a refractory tube below the slide gate plate assembly which is submerged in the molten metal. The refractory tube is referred to as a submerged entry nozzle. One form of slide gate plate assembly employs three plates: an upper plate affixed to the upper tundish nozzle, an apertured middle plate and a lower plate that is connected to the submerged entry nozzle. The flow of molten metal from the tundish through the upper tundish nozzle and into the submerged entry nozzle is regulated by sliding the middle plate so as to close or open its aperture. When the aperture of the middle plate is open, molten metal travels into the submerged entry nozzle and into the mold, whereas as it is closed it throttles molten metal flow. All of the components that come into contact with the molten metal are made of a refractory composition.
Another form of slide gate assembly includes two plates: an upper plate connected to the upper tundish nozzle and a movable gate plate to which the submerged entry nozzle is attached. In the case of the three plate gate assembly, the nozzle is stationary in the mold (the middle gate plate being movable), whereas in the two plate gate assembly the submerged entry nozzle is attached to the gate plate and moves along with it. Additional plates or nozzles may be used with the plate assemblies.
A slide gate plate assembly may also be used under a ladle. The assembly may include two or three plates and is very similar to the tundish gate plate assembly. In use on a ladle, the lower nozzle may be referred to as a shroud and feeds the tundish below it.
Aluminum is added to the steel composition to remove oxygen. While this may reduce or eliminate oxygen, it also has the undesirable effect of possibly clogging the passages of the submerged entry nozzle with accretions of refractory aluminum oxide. In conventional casting methods, nitrogen gas, argon gas, or a mixture of these gases is injected into various locations of the molten metal flow passage such as in the submerged entry nozzle, to scrub the build up of accretions of aluminum oxide on the inside of the passages and to prevent nonmetallic inclusions from adhering inside the passageway.
The system is designed to prevent air aspiration that leads to formation of the refractory deposits along the passageway and clogging. One way this is accomplished in the slide gate assemblies of both the tundish and ladle, is to employ a groove in the lower plate above the lower elongated nozzle (e.g., the submerged entry nozzle). Graphite containing nitrogen gas is injected into the groove periodically to seal the gap between the lower plate and lower nozzle in an attempt to avoid air aspiration. In the case of both the two and three plate gate assemblies, the lower plate remains in place as the submerged entry nozzle exchange apparatus periodically replaces nozzles. The graphite injection feature is used in conjunction with quick nozzle exchange mechanisms.
Conditions under which the graphite containing nitrogen gas is injected are conventionally determined by measuring the back pressure of the gas in the groove. Back pressure may be created by connecting to the groove a tubular coil through which the gas must pass after leaving the groove. Due to surface irregularities and roughness between the refractory material of the top of the elongated lower nozzle (e.g., the submerged entry nozzle) and the bottom of the bottom plate, air is still aspirated into the molten metal passageway, leading to clogging of the lower nozzle. In addition, non-metallic inclusions occasionally form in the molten metal due to air aspiration. When a slab produced by the continuous casting process is rolled into a thin strip, nonmetallic inclusions therein are lengthened, forming "slivers" which may require downgrading or scrapping of the steel containing the slivers. The problem of sliver formation is significant and not avoided by current graphite injection processes and back pressure monitoring or by making the abutting lower plate and lower nozzle surfaces very smooth in an attempt to decrease the gap.
Another persistent problem is the clogging of the nozzle below the graphite injection groove (e.g., the submerged entry nozzle). Such clogging requires occasional "rodding" by workers in which a long rod is rammed through the molten metal passageway to break up deposits of refractory therein. This is a hazardous process and disadvantageous in that dislodged accretions may find their way into the molten metal, which may require downgrading of the steel.
The continuous casting process would benefit from a system that prevents air aspiration between the bottom plate and the submerged entry nozzle, thereby producing better quality steel by reducing the instances of formation of "slivers" in the steel, and reducing the need for "rodding."